The present invention is directed generally to seals, and more particularly, to a packoff assembly for sealing an annulus between an inner tubular member and an outer tubular member. The present invention is especially useful as a casing hanger packoff assembly for an underwater oil or gas well which seals the annular space between a casing hanger and the housing of a subsea wellhead.
In the past, subsea wellhead systems having working pressures of up to 15,000 psi have been known and used in the drilling of underwater wells for the production of oil and gas. An example of one subsea wellhead system having such a 15,000 psi working pressure utilizes a separately installable landing shoulder insert or support member for multiple concentric casing strings and hangers in order to allow full bore access, without underreaming, below the wellhead housing for a standard 171/2 inch drill bit prior to installation of the multiple concentric casing strings. Such a system is disclosed, for example, in U.S. Pat. No. 4,615,544, issued Oct. 7, 1986, which is incorporated in its entirety herein by reference. The multiple concentric casing strings, or surface casing, may be, for example, 133/8 inch, 95/8 inch, and 7 inch strings, all supported on the landing shoulder insert attached to the wellhead housing and concentrically disposed within a conductor casing string, typically a 20 inch string welded to the bottom of the wellhead housing. Each of the surface casing strings is suspended from a hanger, and the hangers are stacked one upon the other, with the uppermost hanger suspending the smallest diameter casing string and the lowermost hanger suspending the largest diameter casing string.
One major problem that arises in 15,000 psi working pressure subsea wellhead systems is to provide a sealing means between the casing hangers and wellhead which will withstand and contain the working pressure. It is an object of the present invention to provide for such a sealing means that is simple, easy to manufacture, easy to install and retrieve, and reliable. It is another object of the present invention to provide for such a sealing means that has minimal requirements concerning externally applied loading force to set the seal. It is yet another object of the present invention to provide for such a sealing means that will be pressure-energized in service up to full working pressure after application of the minimal external loading force. It is also an object of the present invention to provide a sealing means with combined metal and elastomer sealing members to enable the operator to initially load the sealing means only to a point of establishing an elastomer seal with metal back-up rings and thereafter allowing pressure-energization, which may or may not also establish a metal-to-metal seal, depending upon the magnitude of the load experienced in service, or to initially load the sealing means to a point of establishing an elastomer and a metal-to-metal seal and thereafter allowing additional pressure-energization of both the elastomer and the metal-to-metal seals. It is yet another object of the present invention to provide for such a sealing means having improved self-centering characteristics in instances where the casing hanger may have landed slightly off center in the wellhead housing. Still another object of the present invention is to provide for such a sealing means wherein its components may be rotated with respect to one another when required, such as by a failure of the bearing between the packing nut member and the sealing means. It is a further object of this invention to provide a sealing means having a continuous metal link therethrough to provide high tensile strength capacity for those times when it may be necessary to retrieve the sealing means. It is also an object of the present invention to provide a sealing means with the ability for the seal compression to continue after either the inside or outside seal member has reached its maximum ability to compress and the other seal member requires some additional compression.
The sealing means of the present invention accomplishes the above objectives and can be used to reliably seal the annular area between a casing hanger and subsea wellhead housing when the sealing means is energized and experiences a working pressure from above or below of up to 15,000 psi. The sealing means can be energized through the application of less than about 15,000 ft.-lbs. of torque through the drill string, or the equivalent thereof in the case of hydraulic and/or weight setting, and may even be energized with as little as about 1,500 ft.-lbs. of torque or the equivalent thereof, followed by additional pressure-energization in service. A casing hanger packoff assembly of the present invention is adapted to be disposed, for example, on each surface casing hanger of the subsea wellhead system disclosed in U.S. Pat. No. 4,615,544 and to seal the annular space between such hanger and the subsea wellhead housing.